Heat shrinkable labels, which are prepared by printing images and the like on a base film made of heat shrinkable resin, are attached to various containers such as PET bottles and metal cans these days.
For such heat shrinkable labels, polystyrene resin films are often used because of their excellent low-temperature shrinkability. Polystyrene resin films, however, have insufficient heat resistance and insufficient solvent resistance. To solve these problems, use of polyester resin films excellent in heat resistance and solvent resistance are attempted. Polyester resin films, however, are poor in low-temperature shrinkability and rapidly shrink. This easily causes heat shrinkable labels to suffer creases upon attaching the labels to containers. Many heat shrinkable labels have perforations so as to be easily peeled off from containers after use for container recycle, but polyester resin films are difficult to tear along the perforations.
To solve such problems, multilayer films including front and back layers each containing a polyester resin, and an interlayer containing a polystyrene resin are developed, for example. In producing multilayer films, prevention of delamination between layers is important.
One of the measures for preventing delamination between layers is to form adhesive layers between front and back layers and an interlayer. Patent Literature 1 discloses a heat shrinkable multilayer film including adhesive layers formed of an adhesive resin containing a mixture of a polyester resin and a polystyrene resin. However, such a heat shrinkable multilayer film has low adhesive strength between front and back layers and an interlayer, in other words, insufficient adhesive strength between layers.
Patent Literature 2 discloses a heat shrinkable laminate film including adhesive layers formed from an adhesive resin such as a flexible polystyrene resin with a styrene content of 10 to 50%, a modified styrene resin containing a large amount of an elastomer component, a resin that has high compatibility with polyester and thus is compatible with polyester, or a mixture of these resins.
Such heat shrinkable laminate films, however, reduce the adhesive strength between front and back layers and an interlayer after printing images on the films for producing labels, resulting in insufficient adhesive strength between layers.
Patent Literature 3 discloses a heat shrinkable multilayer film including adhesive layers formed from a polyester elastomer.
Such a heat shrinkable multilayer film, however, has white creases, which are caused by folding the film with a strong pressure in a center sealing process in producing a heat shrinkable label. The creases remain even after the heat shrinkable label is placed around a container and then heat shrunk, which spoils the appearance of the film.